Sleep patterns and Seasonal Affective Disorder (SAD) have been scientifically linked for the first time. A mutated gene, PER3, has been found to cause both conditions, and confirms the relationship between unusual sleep timings and depression.

The exact reason why this gene affects mood so much is still unknown, but the research, published in Proceedings of the National Academy of Sciences, is the first to find a mechanism connecting sleep and mental health. The researchers from the University of California believe this is a major stepping stone in the search for new SAD treatments.

SAD is a condition that drives depression in the late autumn and winter months, due to a lack of natural sunlight. It affects roughly 3% of all UK citizens, but in much higher or lower latitudes – where daylight is extremely reduced – this can reach as much as 9%.

"This is the first human mutation directly linked to seasonal affective disorder, and the first clear sign of a mechanism that could link sleep to mood disorders," said Louis Ptáček, a researcher who worked on the study.

Scientists discovered the mutated gene causes unusual sleep patterns and enhanced seasonal depression by testing the effects of the gene on mice, after analysing a family that suffered from obscure bedtimes and depression when daylight is at its shortest.

The family of three all went to bed at 7.30pm on a regular basis, and woke up as early as 4am. All three members experienced SAD annually, and the researchers discovered the PER3 gene was mutated in all three people.

They took that knowledge one step further, and analysed the effects of the mutated gene in mice. They introduced the gene into the mice and altered the 'daylight' the mice experienced by darkening its living quarters.

The mice did not show any symptoms of depression when the daylight and night-time was split 50/50. However, they showed plenty of depression-like symptoms when they were only given four hours of sunlight, such as immobility.

Similarly to humans, when the mice were given antidepressants during this period, the symptoms once again disappeared. This finding was the final step to confirming the link.

"Humans in the short day-lengths of winter develop a mood disorder, and the mice recapitulated that," said Ptáček. "It's quite a dramatic finding."

The next stage for the research is to discover what causes the mutated gene to impact sleep and mental health on a genetic scale. They plan to test the brains of mice, focussing on the chemical pathways which control mood and depression.

Ying-Hui-Fu, co-author of the study, summarised: "It's an exciting time. People have known for decades that light and mood were linked, but this is our first real finger-hold on the problem."